The present invention relates generally to antennas and associated methods for communication and, more particularly, to antennas systems having active polarization correction and associated communication methods.
Antennas are widely utilized in order to transmit and receive a variety of signals. For example, antennas are widely utilized in radio frequency communication systems. Radio frequency antennas are commonly capable of simultaneously transmitting and/or receiving signals having different polarizations, such as orthogonally polarized signals, in order to increase the transmission and/or reception capacity of the antenna. In order to effectively transmit and/or receive signals that are orthogonally polarized, an antenna must have relatively high polarization purity such that there is minimal interference between the orthogonally polarized signals. In some applications, for example, the required cross-polarization isolation may be 30 dB or more.
One common type of antenna utilized for high-data rate Communications with moving platforms is a phased array antenna. Among other advantages, phased array antennas are capable of communicating simultaneously with two or more spatially separate sources. In addition, phased array antennas are relatively easy to install, operate and maintain on moving platforms such as aircraft, ships and motor vehicles since they generally have a relatively low profile, are capable of rapidly tracking and have no moving parts.
Phased array antennas generally include a number of identical radiating elements and a beam former connected to the radiating elements. Each element may include a phase shifter and/or a time delay circuit. In addition, each element may include an amplifier, if desired. In one phased array antenna, each element includes a phase shifter and groups of elements are interconnected by a time delay circuit. By adjusting the phase shift of each element and the time delay of each group of elements, the beam transmitted and/or received by the phased array antenna may be formed electronically and steered without physical movement of the antenna aperture over a wide instantaneous bandwidth. Moreover, by incorporating multiple beam formers and multiple phase shifters and time delay circuits associated with each radiating element, a phased array antenna that is capable of forming multiple simultaneous independent beams may be constructed.
Phase array antennas are capable of transmitting and/or receiving signals having any desired polarization. In this regard, a schematic representation of the architecture of a phased array antenna capable of sensing signals having either circular polarization or arbitrarily oriented linear polarization is shown in FIG. 1. A phased array antenna having the architecture depicted in FIG. 1 includes a plurality of modules 1, each of which includes two amplifiers 2 connected to the orthogonal radiating elements 3. The output of each amplifier connected to a 90xc2x0 hybrid 4 which forms left circular polarization out of one port and right circular polarization out of the other port. Each port is connected to a phase shifter 5 which, in turn, is connected to an independent beam forming network 6. The output of each beam former is therefore a left or right circularly polarized (CP) beam that is redirected independently of the other beam. A phased array antenna having the construction depicted in FIG. 1 may also operate in a linearly polarized (LP) mode. In this mode, the two beams are co-pointed and the beam former outputs are recombined in a quadrature hybrid 7 to recover two orthogonal linear polarizations from a single source. By controlling the two phase shifts 5 to have a constant offset therebetween, the two orthogonal linear polarization axes may be spatially rotated so as to be aligned with the polarization axes of a source, such as a satellite that radiates orthogonal linearly polarized signals Although phased array antennas offer a number of advantages, phased array antennas, in particular, and electronically scanned antennas, in general, are typically unable to provide the degree of polarization purity over the entire range of scan angles as that provided by at least some mechanically scanned antennas. The limitations with respect to the polarization purity of electronically scanned antennas are created by construction constraints within the modules, inherent radiating element cross-polarization characteristics and the active impedance to which a module is subjected once a module is placed in an array. Notably, this disadvantageous cross-polarization coupling between signals having orthogonal polarizations is within the antenna itself and is independent of any cross-coupling between signals having orthogonal polarizations that may occur in the propagation medium.
For signals transmitted and/or received in a near broadside direction, the cross-polarization isolation is determined largely by the degree of cross-coupling between orthogonal radiating elements. While phased array antennas can be constructed with near broadside polarization isolation approaching that of mechanically scanned antennas, the cost of the modules that must be constructed generally increases substantially. Unfortunately, as the scan angle increases away from broadside, the cross-polarization isolation degrades due to divergence between the E and H-plane active impedances seen by each module in the array. The degree of divergence typically increases monotonically with elevation scan and varies smoothly and periodically with azimuth scan. At an elevation scan of 60xc2x0, for example, the degree of degradation of the cross-polarization isolation relative to that provided near broadside will vary as the antenna is scanned in an azimuthal direction by as much as 10 dB. The internal coupling between the orthogonally polarized signals within an antenna 20 may be graphically depicted as shown in FIG. 2. In this regard, the antenna is represented by the combination of two blocks, one block 22 depicting an ideal antenna having no internal coupling between the orthogonally polarized signals and another block 24 depicting the internal coupling between the orthogonally polarized signals. As will be apparent, although the antenna is depicted for purposes of discussion as being separated into two boxes, the antenna cannot physically be separated in the same manner as the internal coupling between the orthogonally polarized signals is inherent within the antenna as a result of its construction and design.
Referring to FIG. 2, the antenna 20 includes a pair of terminals 26 and at least one pair of orthogonally polarized radiating elements 28. In the transmission mode, two orthogonally polarized signals T1 and T2 are presented at the antenna terminals and are amplified by the ideal antenna 22 by a gain designated A. These amplified signals are then subjected to undesirable cross-polarization coupling as represented by block 24. As indicated within block 24, the ratio of the cross-coupled voltage to the signal voltage is designated xcex4. As such, the signals transmitted by the dual orthogonally polarized radiating element are not merely the amplified inputs designated AT1 and AT2, but are instead more complex signals in which each signal includes components having both polarizations. In the illustrated example, the radiating element designed to radiate signals having the first polarization p1 actually radiates a signal defined as A[T1(1xe2x88x92xcex42)1/2{circumflex over (p)}1+xcex4T2{circumflex over (p)}2], while the radiating element designed to radiate signals having the second polarization p2 actually radiates a signal represented as A[xcex4T1{circumflex over (p)}1+(1xe2x88x92xcex42)1/2T2{circumflex over (p)}2].
Similarly, in the reception mode, dual orthogonally polarized signals are presented to the dual orthogonally polarized radiating elements 28 as designated R11 and R22. Instead of being merely amplified by the antenna and presented at the antenna terminals 26, the internal cross-polarization coupling causes each of the signals to include components having both polarizations. As such, the signals actually presented to the antenna terminals are represented as A[R1(1xe2x88x92xcex42)1/2+xcex4R2] and A[xcex4R1+(1xe2x88x92xcex42)1/2R2].
Several techniques have been developed to improve the polarization purity of phased array antennas at high scan angles. Each technique attempts to improve the polarization purity in a passive manner and is somewhat effective, although practical limitations generally prevent the desired degree of polarization purity from being achieved.
One technique to improve polarization purity is to reduce the spacing between radiating elements. The degree to which the spacing between radiating elements may be reduced is limited, however, by packaging issues and by a loss in the gain that the radiating elements may provide that results from a decrease in the spacing. In this regard, the area available in which to package the phase shift and amplification circuitry decreases proportional to the square of the decrease in element spacing. The spacing between radiating elements for wide scan performance must be less than 0.577xcex, where xcex is the wavelength of the signals transmitted and/or received by the antenna. Particularly at frequencies above 10 GHz, however, it is difficult and expensive to appreciably decrease the spacing between the radiating elements to less than 0.577xcex, especially in packaging schemes that are relatively thin and planar. By incorporating a vertical packaging architecture, the spacing between the radiating elements may be decreased somewhat more, but only at the expense of a generally disadvantageous increase in the thickness of the antenna. As the radiating elements become more closely spaced, the gain which each radiating element is capable of providing also decreases. This decrease in the gain that each radiating element is capable of providing may require that the number of radiating elements be increased in order to provide the same overall intended performance, i.e., Gain to noise temperature (G/T) for reception antennas and Effective Isotropic Radiated Power (EIRP) for transmission antennas. As will be apparent, an increase in the number of radiating elements correspondingly increases the cost of the antenna and, with respect to phased array antennas, also increases the power consumption of the antenna. Accordingly, high polarization purity cannot be obtained as a practical matter at high scan angles simply by reducing the spacing between the radiating elements.
Another technique to improve the polarization purity of electronically scanned antennas, such as phased array antennas, is to provide a wide angle impedance (WAIM) layer that is disposed over the radiating elements. A WAIM layer is constructed from a plurality of dielectric layers that serve to improve the cross-polarization isolation at relatively high scan angles. Unfortunately, the number of dielectric layers that would be required in order to effectively suppress cross-polarization coupling over an azimuth scan of 360xc2x0 far exceeds the number that may be practically employed.
Even those phased array antennas, having a relatively small spacing between the radiating elements and including a WAIM layer are unable to provide sufficient cross-polarization isolation at some of the scan angles for some applications. For example, a phased array antenna capable of providing cross-polarization isolation in the broadside direction approaching 28 dB may generally only be able to provide cross-polarization isolation of about 15 dB at an elevation scan angle of 60xc2x0 and at the worst case azimuth angle. As such, it would be desired to provide an antenna system which provides enhanced polarization isolation between signals transmitted and/or received having orthogonal polarizations.
An antenna system and an associated method are provided that are capable of providing improved cross-polarization isolation, thereby negating the otherwise deleterious effects of cross-coupling between orthogonally polarized signals that occur within a dual orthogonally polarized antenna, such as a phased array antenna. Thus, the antenna system can more reliably transmit and/or receive dual orthogonally polarized signals over a wide range of elevation and azimuth scan angles.
The antenna of one advantageous embodiment includes a dual orthogonally polarized antenna, such as a phased array antenna, capable of supporting propagation of signals having two orthogonal polarizations. The antenna permits different predetermined amounts of internal coupling between the orthogonally polarized signals at different scan angles. The antenna system of this embodiment also includes a cross-polarization cancellation element associated with the antenna for modifying the orthogonally polarized signals to compensate for the internal coupling. The cross-polarization cancellation element modifies the orthogonally polarized signals at different scan angles based on the different predetermined amounts of internal coupling between the orthogonally polarized signals at different scan angles.
The antenna system may also include a processor for directing the cross-polarization cancellation element to provide appropriate modifications to the orthogonally polarized signals and a memory device, accessible by the processor, for storing data representing modifications to be provided by the cross-polarization cancellation element to the orthogonally polarized signals at different scan angles. In addition to variances based upon the scan angle, the data stored by the memory device may also be dependent upon the frequency of the orthogonally polarized signals.
In embodiments in which the dual orthogonally polarized antenna is a phased array antenna, the phased array antenna may include a pair of input/output ports for providing signals having a respective polarization. As such, the cross-polarization cancellation element of one embodiment may be connected to the pair of input/output ports. The phased array antenna also includes a plurality of modules, each including a pair of dual orthogonally polarized radiating elements. As such, the cross-polarization element need not be connected to the pair of input/output ports of the phased array antenna. Instead, the antenna system of another embodiment may include a plurality of cross-polarization cancellation elements associated with respective modules of the phased array antenna.
In one embodiment in which the cross-polarization cancellation element is connected to the input/output ports of one antenna, the cross-polarization cancellation element includes a first leg extending from the first input/output port to the second input/output port, and a second leg extending from the second input/output port to the first input/output port. Each leg includes an adjustable amplifier and an adjustable phase shifter for controllably adjusting the amplitude and phase, respectively, of the signals diverted from one input/output port to the other input/output port. By appropriately adjusting the amplitude and phase, the internal coupling between the orthogonally polarized signals that is permitted by the antenna may be corrected.
According to another embodiment in which the cross-polarization cancellation element is connected to the input/output ports of the antenna, the cross-polarization cancellation element includes first and second legs connected to the first and second input/output ports, respectively. Each leg includes an adjustable, i.e., variable gain, amplifier or an adjustable attenuator (hereinafter generally termed an adjustable amplifier) and an adjustable phase shifter for controllably adjusting the amplitude and phase, respectively, of the signals at the respective input/output port. In addition, the first and second legs of the cross-polarization cancellation element may include at least one quadrature hybrid connected between the legs. By appropriately adjusting the amplitude and phase of the signals, the cross-polarization cancellation element may similarly compensate for the internal coupling between the orthogonally polarized signals. Thus, the antenna system of the foregoing embodiments provide open loop control of the cross-polarization coupling.
The antenna system of another embodiment includes both a reception antenna for receiving signals having two orthogonal polarizations and a transmission antenna for transmitting signals having two orthogonal polarizations. Both the reception antenna and the transmission antenna may be phased array antennas. In addition, both the reception antenna and the transmission antenna permit internal coupling between the orthogonally polarized signals. The antenna system of this embodiment also includes first and second cross-polarization cancellation elements associated with the reception and transmission antennas, respectively, for modifying the orthogonally polarized signals to compensate for the internal coupling. Each cross-polarization cancellation element includes a delta port and a sum port. In addition, each cross-polarization cancellation element includes at least one phase shifter for modifying the phase of at least some of the orthogonally polarized signals. Each cross-polarization cancellation element may also include an adjustable amplifier for controllably adjusting the amplitude of at least some of the orthogonally polarized signals.
According to this embodiment, the antenna system also includes a processor for setting the phase shift and the amplitude imparted by at least one phase shifter and the amplifier of the first cross-polarization cancellation element, respectively, typically in an iterative manner, such that a null is provided at the delta port. The processor then sets at least one phase shifter and amplifier of the second cross-polarization cancellation element to impart the same phase shift and amplification, respectively. Thus, the antenna system of this embodiment effectively employs closed loop feedback in order to compensate for the internal coupling between the orthogonally polarized signals permitted by the reception and transmission antennas.
The antenna of this embodiment may include first and second input/output ports for providing signals having a respective polarization. As such, each cross-polarization cancellation element is connected to the first and second input/output ports of the respective antenna. Each cross-polarization cancellation element may include the first and second legs connected to the first and second input/output ports, respectively, of the respective antenna. Each leg includes an adjustable amplifier and an adjustable phase shifter for controllably adjusting the amplitude and phase, respectively, of the signals at the respective input/output port. The first and second legs of each cross-polarization cancellation element of this embodiment may also include at least one quadrature hybrid connected therebetween such that the cross-polarization cancellation element may compensate for the internal coupling between the orthogonally polarized signals that is permitted by the respective antenna.
In operation, orthogonally polarized signals may be received by the reception antenna with the phase of the received signals then being selectively shifted and, in advantageous embodiments, the amplitude of the received signals also being selectively adjusted such that a null is provided at the delta port of the cross-polarization cancellation element. The phase shift and, in advantageous embodiments, the amplitude adjustment imparted upon the transmitted signals is then set to be equal to the phase shift and the adjustment in amplitude imparted upon the received signals. The orthogonally polarized signals may then be transmitted via the transmission antenna with the phase shift and amplitude adjustment compensating for the internal coupling between the orthogonally polarized signals that is permitted by the transmission antenna.
Accordingly, the antenna system and associated method of the present invention provide active correction for internal coupling between the orthogonally polarized signals permitted by the dual orthogonally polarized antenna, such as a phased array antenna. As such, the antenna can transmit and/or receive dual orthogonally polarized signals across a full range of scan angles without concern as to the degradation of the polarization purity of the signals. Additionally, the antenna system of the present invention provides active polarization correction without substantially increasing the cost of the antenna system and without significantly impacting the size or packaging requirements of the antenna. Nevertheless, the antenna system of the present invention can be used with any of the conventional passive measures described herein, to provide additional polarization purity.